However, this trend got murky over the past decade, with satellite data suggesting that the sea level has risen slightly more slowly in the past decade than in the previous one.

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That would be good news if true. Yet the result was odd because other studies showed that more water than ever was pouring into the oceans from melting glaciers and ice caps.

“It was a bit of puzzle,” says Christopher Watson of the University of Tasmania in Hobart.

Satellite errors

One possible explanation was that more water had been accumulating on land because of increased rainfall in some places. This can undoubtedly cause fluctuations in sea level; the intense flooding in Australia and elsewhere in 2011, for instance, caused a distinct dip.

Watson’s team has now shown the rates have not actually slowed down but are, indeed, still accelerating. It found that the apparent slowdown was due to errors in satellite data, which have been used since 1993 to measure sea levels, in addition to tide gauges.

His team identified errors in the satellite record by, among other things, comparing the satellite data to a larger number of tide gauges than any previous analysis. Their analysis shows the apparent decline was due to calibration errors that meant the first satellite – Topex A, which operated from 1993 to 1999 – slightly overestimated sea levels. This masked the ongoing acceleration.

The results fit in much better both with measurements of ice loss and with projections of future sea level rises. “It is consistent with all the projections,” Watson says.

Not comparable

Richard Alley of Pennsylvania State University, says this is a thorough and careful study. “Considering how hard these measurements are, it is not surprising that a new study such as this one finds an additional small correction,” he adds.

Unfortunately, the study doesn’t tell us what we really need to know – how rapidly the sea level will rise over the next century. The satellite record is so short that the observed acceleration is not statistically significant and does not allow us to compare it with existing projections.

According to the last Intergovernmental Panel on Climate Change (IPCC) report in 2013, sea levels could rise by up to 1.2 metres by 2100 without drastic cuts in greenhouse emissions – and, so far, emissions are climbing faster than ever.

Grim findings

The IPCC figure, though, is based on the questionable assumption that there will be no sudden break-up of the great ice sheets. Some researchers expect the break-up to happen faster, so the sea level could rise by 2 metres or even more by 2100.

What’s more, since the IPCC report was written, there has been a series of very grim findings.

Last year, we heard that the collapse of part of the West Antarctic ice sheet containing enough water to raise the sea level 1.2 metres could not be stopped. This year it emerged that part of the East Antarctic ice sheet, which was not expected to lose much ice for many centuries, may be unstable too. It holds enough ice to raise the sea level by 3.2 metres.

“The new science seems to be strengthening the possibility of ‘much worse’,” says Alley. In other words, we are on course for a rise in the sea level of many metres.

That means many low-lying coastal areas and cities are doomed.

What is not clear yet is whether it will take centuries for the sea to drown them or whether it could happen much sooner. Most think it will be many centuries, but there is no way to be sure. “We cannot exclude the possibility of an abrupt change, a ‘drunk driver’ in the climate system,” says Alley.